The present invention relates to a laminated structure and a manufacturing method thereof. Particularly, the present invention relates to a laminated structure having a coating layer suitable for an optical disk recording medium, and a method of manufacturing the laminated structure.
As for read-only type optical disks of which CD-ROMs and DVD-ROMs are representative, one method of increasing the recording capacity per optical disk is a method of forming a recording layer, which includes signal information, as a two-layer structure. In a method in which a laser is irradiated from one side of an optical disk and what is recorded in the disk is thereby read in the same direction, of two layers, the layer closer to the side on which the laser is irradiated needs to be a semipermeable recording layer having some degree of light transmittance and light reflectance. Conventionally, this semipermeable recording layer is obtained by using gold or silver as a target material in a sputtering method and depositing it onto a substrate having convex and concave pits. The semipermeable recording layer formed of gold or silver has a light transmittance and a light reflectance necessary for the optical disk.
However, the semipermeable recording layer of this type has a disadvantage in terms of manufacturing cost since gold or silver is expensive, and a problem of difficult handling arises because the specific gravity of the semipermeable recording layer is large. Therefore, as a material for the semipermeable recording layer of the optical disk, there has been a demand for a material to replace gold or silver. Moreover, in the case of the semipermeable recording layer, since a thin layer of pure gold or silver is formed during sputtering, there is no way but to control the thickness of the semipermeable recording layer in order to obtain light reflectance and the like necessary for the semipermeable recording layer, and a problem arises that the semipermeable recording layer is not easily formed.
In general, sputtering is used to form the semipermeable recording layer. One of the reasons why sputtering is used is that, in order to lower the formation cost, physical methods such as sputtering, ion plating, vacuum deposition, and the like are advantageous over chemical methods of which a CVD method is representative, and among these physical methods, sputtering enables high-speed formation of the semipermeable recording layer and excellent adhesion of the semipermeable recording layer with respect to the substrate. In the case of using the DC power system by which the semipermeable recording layer can be formed at high speed, the target material used for sputtering needs to have such an electric conductivity that the volume resistivity is 100 xcexa9xc2x7cm or less, and preferably about 102 xcexa9xc2x7cm or less.
An object of the present invention is to overcome the conventional problems described above, that is, to provide a laminated structure having a silicon carbide coating layer which has excellent oxidation resistance, chlorine resistance, moisture resistance, and the like, and is therefore suitable for an optical disk recording medium such as a CD-ROM, a DVD-ROM, and the like, in particular, and to provide a method of manufacturing a laminated structure in which the aforementioned laminated structure can be simply and reliably manufactured.
As a result of their thorough investigations for the purpose of achieving the above object, the present inventors have learned the following. As a result of seeking a substitute material for gold or silver as a target material used in the sputtering method, they have found that metals other than gold or silver are difficult to handle because many of them have a large specific gravity in general. Further, since a thin layer of a pure metal is formed by sputtering, the only way to obtain the light reflectance and the like necessary for a semipermeable recording layer is to control the thickness of the semipermeable recording layer, and manufacture thereof is thus not easy. On the other hand, many ceramic materials are generally insulating materials, and their use as target materials is difficult. However, a sintered silicon carbide used in the present invention has a volume resistivity of 100 xcexa9xc2x7cm or less, and therefore is effective as a target material which can control, by sputtering conditions, the optical characteristics of the semipermeable recording layer formed in a thin shape.
Further, as the target material which can control the optical characteristics of the semipermeable recording layer by sputtering conditions, a sintered silicon carbide of high density and high purity having a volume resistivity of 100 xcexa9xc2x7cm or less is further effective. The sintered silicon carbide is obtained by carrying out, in a process for producing a sintered silicon carbide disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-67565 proposed by the present inventors, a step of introducing a nitrogen-containing compound at the time of preparing a mixture of a silicon carbide powder and a non-metallic sintering assisting material, or a step of introducing a nitrogen-containing compound during preparation of a silicon carbide powder at the time of mixing a carbon source and a silicon source, which are the raw materials of the silicon carbide powder. Furthermore, a sintered silicon carbide of high density and high purity, having an impurity ratio of less than 1.0xc3x971011 atoms/cm2 on or near the surface, and having a volume resistivity of 100 xcexa9xc2x7cm or less, is further effective. This impurity ratio on or near the surface is obtained by cleaning processes disclosed in Japanese Patent Application Nos. 10-348569 and 10-348701 proposed by the present inventors.
Based on the above-described knowledge obtained by the present inventors, the present invention includes the following aspects.
 less than 1 greater than  A laminated structure which has a substrate formed of a synthetic resin or glass and a silicon carbide coating layer formed on the substrate by sputtering in which a sintered silicon carbide having a volume resistivity of 100 xcexa9xc2x7cm or less is used as a target material, wherein the silicon carbide coating layer has a light transmittance of 80% or less and a volume resistivity of 100 xcexa9xc2x7cm or less.
 less than 2 greater than  A laminated structure according to  less than 1 greater than , wherein the silicon carbide coating layer has a light reflectance of 10 to 50%.
 less than 3 greater than  A laminated structure according to  less than 1 greater than  or  less than 2 greater than , wherein the synthetic resin is polycarbonate.
 less than 4 greater than  A laminated structure according to any of  less than 1 greater than  to  less than 3 greater than , wherein the impurity ratio on the surface of the silicon carbide coating layer is 1.0xc3x971012 atoms/cm2 or less.
 less than 5 greater than  A laminated structure according to any of  less than 1 greater than  to  less than 4 greater than , wherein the thickness of the silicon carbide coating layer is 15 to 100 nm.
 less than 6 greater than  A method of manufacturing a laminated structure having a substrate formed of a synthetic resin or glass and a silicon carbide coating layer formed on the substrate by sputtering in which a sintered silicon carbide having a volume resistivity of 100 xcexa9xc2x7cm or less is used as a target material, wherein the silicon carbide coating layer is formed by controlling electric power inputted to a sputtering device, flow rate of oxygen gas or nitrogen gas introduced, and sputtering time.
 less than 7 greater than  A method of manufacturing a laminated structure according to
 less than 6 greater than , wherein the target material is a sintered silicon carbide, and the sintered silicon carbide has an impurity ratio of 1.0xc3x971012 atoms/cm2 or less on and near the surface thereof, and a density of 2.9 g/cm3 or more.
 less than 8 greater than  A laminated structure according to any of  less than 1 greater than  to  less than 5 greater than , wherein the density of the silicon carbide coating layer is 2.9 g/cm3 or more.
 less than 9 greater than  A laminated structure according to any of  less than 1 greater than  to  less than 5 greater than  and  less than 8 greater than , used as an optical disk recording medium.